The waft of acrid air, followed by the virtually instantaneous cessation of forced air, served as conclusive evidence that the bell had tolled for the long-serving, slow-turning, labored heater fan in my BMW. Of all the maintenance and repair tasks required of a hands-on vintage-car enthusiast, replacing the heater fan and/or heater core is among the worst. Think back to all of those old assembly line photos, and you'll notice that of all the interior parts the heater box goes in first--or, at the very least, before everything else that gets in its way.
There was no point diagnosing the situation or repeatedly replacing the fuses. I've been around old 2002s long enough, particularly those which have sat unused for extended periods of time, to know that there is no use. The heater fan in a BMW 2002 sits just below the center of the windshield, only marginally protected from the elements. With the inevitable moisture, dirt, use, a heater fan on these cars (and many others) have a relatively hard life.
Because the heater assembly goes in the car early on, removing it requires removing a mountain of parts just to gain access. Later BMWs, such as the E30 and E36, have it much easier and require only a few bolts and clamps to gain access. With the number of man hours required, this was going to be a do-it-yourself job.
In basic terms, it's not really all that bad. Remove center console, remove lower kick panels, remove heater core hoses, detach bowden cable mounts from dash and remove heater box assembly.
I was wrong.
The particular car shown had at one time had its dash replaced, not with the same type of two-piece dash, but with a three-piece dash from an earlier model. I discovered this as I attempted to remove the lever covers on the heater controls. The early dashes had them screwed in from behind. This requires removing the entire dashboard. On these dashes, there are two screws under the windshield seal, requiring the removal of the windshield (which often breaks).
This was not going well. A call to my mechanic friend at Bavarian Rennsport confirmed the sum of my fears: This sucks.
Not one to be beaten, I was determined to win come hell or high water. Proceed as normal. Follow along as I take an unorthodox approach to solving a problem that cannot be solved.
If you have another older BMW, or any older car for that matter, get a copy of the factory manual and replace the broken heater fan yourself. You will end up with cut-up hands and will have unleashed a torrent of curses, but you will also have saved lots of money.
The testing procedure, which is universal and generic, goes like this.
1: Check for blown fuse.
2: Check to see if fuse is of the right amp rating for the heater circuit.
3: If #2 = yes, then curse.
4: Remember that nothing magically breaks. If something is broken, it won't fix itself, either.
5: Remove two wires at heater fan (after having unburied it from where it fits).
6: For grins, spin the heater fan and check to see that it turns freely. In my case, the fan was reticent to turn, indicating mechanical failure. Another common problem is worn-out brushes inside the motor.
7: Test to see if ground wire has a ground (usually the brown one on German cars).
8: Test to see if positive wire has power when the switch is turned on (temporarily replacing the fuse with a new one).
9: Check the resistance for each of the terminals leading to the heater box and the positive wire attached to the motor. The values for this three-speed motor should be none, low, high for high, medium and low speeds, respectively.
10: If all of the above checks out, check the heater fan switch for signs of arcing or mechanical failure. Having been around 2002s enough, and the fact that the fan was seized, I circumvented the diagnostics and went where experience told me to go (replace fan).
Note that these are simplified diagnostics and won't fit for every application. As always, read the factory manual for your vehicle. Your patience will thank you.
No question about it--excessive load caused the fuse for the heater to break. The slowness of the fan motor in the time leading up to failure was a good indication that something was amiss. | 
With the wires disconnected, the wires to the heater fan deliver voltage in all switch positions. | 
This handy multi-meter was purchased from a wholesale electronics supply house and does most of what is required to work and diagnose car problems. For engines with points, a meter such as this measures dwell and rpm. |
A test lamp can be used to perform many different tasks. Ground the alligator clip and probe with the point to anything you expect to have current. If it doesn't light up, something is broken in the circuit. Even if you don't have a multi-meter, a test light can do important functions such as setting static engine timing and, when used with a power source, act as a continuity tester--all for about $7. | 
It doesn't look too formidable, but the center console must come out. There are a zillion screws, each of which is hard to find new when lost. | 
In theory, these plates pop out, exposing the two screws that need to be removed in order to release the heater controls. Not so in this case. |
I followed the ordinary removal procedure up to a point. At this stage, the heater controls were still mounted to the dash. Note the use of rags to catch antifreeze and the ancient original heater control valve (shown on top of the rag). | 
Once the coolant in the heater core was contained, I removed the three clips at the bottom of the heater box shown in this photo (one has already been removed). This allowed me to reach into the heater box to remove the two left-side bowden cables at their mounting points. The three pop rivets at the top of the heater box could then be drilled out to separate the halves of the heater box. Note that there is a right-hand bowden cable and another going to the heater valve. Factory procedures call for re-assembling the heater box with pop rivets or 4x10mm bolts. I am the proud owner of a new fancy schmancy pop rivet gun. | 
With the heater box in pieces, I could then attend to the internals. The foam seals on the flap doors, long since deteriorated, were replaced with pieces of adhesive-backed insulating foam. It's not pretty, but it's better than nothing. As a side note, Bill at Bavarian Rennsport said that he and his crew replace about one of these heater fans each month, and, without exception, the heater boxes need rebuilding, which he does with new foam and grommets. A wise choice. |
This new motor and fan assembly is made by Bosch, the O.E. supplier. The fan and motor come as a complete assembly. Note that the fan blades may require a bit of tweaking to clear one of the inner flaps. Just be careful you don't bend too much. This piece costs about $200 from Maximillian Importing. Yes, I paid; and, yes, it hurt. | 
Removing the old fan is a simple matter of prying off four clips. The new fan slides into the same space, but alignment is critical due to a recess and tab on the top of the motor that only fits one way. The clips don't lock over the metal portion of the motor housing as with the old; they lock onto the new plastic sleeve surrounding the motor. I suspect Bosch makes the one fan motor but different plastic sleeves for the different applications. | 
Here the heater fan is shown in place, and the two halves of the heater box assemblies are ready to be rejoined for the next 28 years. |
Since it was such a PITA getting everything apart, I replaced the 28-year-old heater control valve with a new one from Behr, the O.E. supplier. | | |